This invention relates to the field of textile structures intended for the production of composite materials. It more particularly relates to a warp and weft fabric produced, for the greater part, from multifilament technical threads with a relatively high yarn count for a relatively low weight per unit area and to a corresponding method for producing the same.
It is known that composite materials have undergone a major expansion, because they combine excellent mechanical properties with low weight. Such materials essentially comprise a textile reinforcement and a resin matrix. Those skilled in the art know that the production of these materials presents some difficulties. In fact, for some uses, in particular in the aeronautical industry, the mechanical properties of the composite materials are strictly defined.
It is often required that the textile structures used in composite materials are sufficiently tightly woven so as to retain a regular geometry and an appropriate handling capacity, while at the same time allowing sufficient penetration of the resin during manufacture of the composite. This enables satisfactory mechanical properties to be obtained in the final composite. It is thus necessary to use sufficiently fine fibers to make such tightly woven structures.
As a function of the desired weight per unit area for the structure, a thread giving perfect covering is chosen, in other words a regular spread which does not leave visible porosities and which, correspondingly, leads to a high volume ratio. It is observed that the lower the weight per unit area of the textile structure, the more the yarn count of the fibers, in other words the linear mass of each fiber, must also be low.
However, fine threads are relatively expensive and this is particularly true for the carbon threads currently available on the market. For example, the price of 1K (1000 filaments) carbon threads is about four times that of 3K threads and six to eight times that of 6K threads. It should be understood that the higher the number of filaments in the threads, the higher the yarn count of the threads.
It is thus advantageous to use coarser threads whose price decreases as the coarseness thereof increases. For example, 6K (6000 filaments) carbon threads, which are twice as coarse as 3K threads, are approximately 30% less expensive. It is the same for 12K threads which are now available on the market and whose price is 30% lower than that of 6K threads.
In order to retain and increase their market share, composite materials must be available at prices lower than those currently in force. In particular in the aeronautical field, it is desirable that the price of a composite component should correspond to that of an aluminum component, which necessitates substantial cost reductions. Since the price of fibers and particularly carbon fibers has a direct effect on the cost of composite components, the choice of the type of fibers is critical.
It is in particular 6K and 12K threads which would enable costs to be reduced. A fabric from 6K threads is about 30% cheaper than a fabric from 3K threads, for the same weight per unit area. A fabric produced from 12K threads is about 50% cheaper than a fabric of the same weight per unit area produced from 3K threads.
However, if fine threads are replaced by threads of a higher yarn count, while keeping the same weight per unit area, for example replacing four 3K threads with one 12K thread, holes generated in the resulting fabric are proportionately larger than for lower weights per unit area.
Coarser threads are thus unsuited for use in textile structures whose weight per unit area or unit area weight is relatively low, when conventional weaving methods are used. Effectively the structures obtained are too open, and in addition they cannot be easily handled as they leave the weaving loom.
The use of coarser threads is therefore currently limited to fabrics with relatively high weights per unit area. An analysis of the balanced carbon fabrics available on the market, in other words those for which the weight of the warp threads is identical to the weight of the weft threads, and having a uniform surface without porosity, leads to a relation between the thread used and the weight per unit area of the fabric.
For example, 1K threads are used for fabric whose weight per unit area is generally between 90 and 210 g/m.sup.2.
Fabrics with a weight per unit area lower than 90 g/m.sup.2 can be produced from 1K threads but their porosity is not compatible with the objective of perfect coverage.
With regard to 3K threads, the weight per unit area of fabrics is generally between 180 and 400 g/m.sup.2 ; for 6K threads, it is generally from 260 to 600 g/m.sup.2, and lastly for 12K threads it is generally between 465 and 800 g/m.sup.2.
The above comment concerning carbon fabrics from 1K threads, relating to the minimum weight of the fabrics, also applies to carbon fabrics obtained from 3K, 6K and 12K threads.
In the textile industry various methods are known for reducing the porosity originally present in a textile structure.
Thus, FR 2 478 693 discloses a method for reducing the porosity of a preimpregnated fabric, and more particularly an impregnated fabric comprising carbon fibers, without the need for finer fibers.
This method consists of successively forming a fiber from filaments having a relatively circular cross section, weaving such fibers to form a fabric having relatively large interstices, impregnating the fabric with a non-cured resin, engaging a cylinder on one side of the impregnated fabric while supporting the other side of the fabric opposite the cylinder, and moving the cylinder on the fabric a sufficient number of times to obtain a desired flattening of the fibers.
This calendaring flattens the fibers so as to reduce the size of the interstices, making it easier for the interstices to be filled when the resin cures and thus reducing the porosity of the finished cured laminate.
However, the production of a dry fabric presents greater difficulties because of the absence of a second material to fill the interstices between the threads.
Nevertheless, within the scope of composite material production, the availability of non-impregnated or dry fabrics is required. This is particularly due to the fact that they can be used very generally, with all types of resin.
EP-0 302 449 discloses a method for reducing the interstices in a fabric. This method was designed for conventional fabrics, produced from fine threads, in particular 3K fibers. It was in fact observed that these fabrics contained porosities which it was necessary to reduce in order to obtain a uniform distribution of the fibers and the resin in the final composite.
Such method does not teach the use of threads with a relatively high yarn count. Such method moreover is not designed for high yarn count threads, since the document mentions that conventional fabrics based on fine threads already contain porosities which adversely affect the properties of the final composite.
It thus seemed advantageous to develop a fabric produced from synthetic threads whose yarn count is relatively high with respect to the weight per unit area of the fabric, the fabric having a porosity or a fiber volume ratio compatible with its use in the manufacture of a composite material having satisfactory mechanical properties.
Throughout the specification, the fiber volume ratio (FVR) has a value defined as follows: ##EQU1## It can be understood that the fiber volume ratio can be calculated at any point in the fabric.
Similarly, throughout the specification "a FVR approximately constant in the fabric" means a FVR whose average value is constant, a local variation of .+-.3% being acceptable.